The production of bacteriophage T4 tRNA Pro and tRNA Ser represents the best understood example of a biosynthetic pathway leading to tRNA. The seven terminal steps involved in the biosynthesis of these tRNAs have been defined by mutant methodology and confirmed by a reconstruction of the reactions in vitro from purified precursor RNA intermediates and enzymes. In this work, we have gained some appreciation of the interactions between a precursor RNA and its processing and modifying enzymes. The next phase of this work will be directed at obtaining a more comprehensive description of the molecular features underlying these interactions. This effort is likely to be assisted by the availability of mutants defective in specific interactions, together with a knowledge of the structural features of the relevant parts of the precursor RNA molecule. An additional effort will be made to extend the biosynthetic pathway back toward the DNA, to include a definition of the steps responsible for nucleotide modifications and possible cleavage reactions that lead to the Pro-Ser precursor RNA.